In recent years, there has been promoted weight reduction of various parts constituting an automobile in order to improve fuel consumption of an automobile. Weight reduction means differ depending on each required performance of the parts, and for example, for a framework part, thickness thinning achieved by increasing strength of a steel sheet is performed, and for a panel part, application of a light metal such as an Al alloy to a steel sheet and the like are performed. However, when compared to steel, the light metal such as an Al alloy is expensive, so that it is mainly applied to luxury automobiles in the real world.
On the other hand, a demand for automobiles is being shifted to emerging countries from developed countries, and from now on, it is expected that weight reduction and price reduction are both achieved. For any parts, it becomes necessary to achieve, of steel, strength increase and weight reduction achieved by thickness thinning.
Aluminum casting and forgings have been advantageous to wheels for passenger vehicles in terms of design. However, even though steel pressed products are used recently as the wheels for passenger vehicles, by devising materials and methods, products having the design equivalent to that of an aluminum wheel are appearing.
Particularly, in addition to excellent fatigue endurance and corrosion resistance that have been required so far in a wheel disc seen by an end user, the design and beautifulness equivalent to those of an aluminum wheel are also required in a steel wheel. Similarly, also in a steel sheet for wheel disc, workability improvement for improving design as a part and surface property improvement for securing beautifulness are required, in addition to the strength increase that achieves thickness thinning, and the fatigue endurance and the corrosion resistance that have been required so far.
As properties that have been required so far in the steel sheet for wheel disc, bulging workability, drawability, and fatigue endurance have been regarded as important in particular. This is because working of a hat portion is challenging among forming steps of the wheel disc and the fatigue endurance is managed by the strictest standard among member properties of the wheel.
At present, in order to emphasize the fatigue endurance of a member as a high-strength hot-rolled steel sheet for wheel disc, ferrite-martensite dual phase steel sheets of 590 MPa grade excellent in fatigue property (what is called Dual Phase steel) have been used. However, the strength level required in these steel sheets is increased to the 780 MPa grade from the 590 MPa grade and the strength tends to further increase.
In Non-Patent Document 1, there has been disclosed a method of securing uniform elongation even with the same strength by turning a microstructure of a steel sheet into a composite-structure such as a Dual Phase steel composed of ferrite and martensite (to be described as DP steel, hereinafter).
On the other hand, the DP steel has been known that local deformability typified by bending forming, hole expansion, and burring is low. This is because a strength difference between ferrite and martensite is large, so that large strain and stress concentration occur in ferrite near martensite with formation and a crack occurs.
Based on this finding, a high-strength steel sheet whose hole expansion ratio is increased by decreasing the strength difference between structures has been developed. In Patent Document 1, there has been proposed a steel sheet in which strength is secured by applying bainite or bainitic ferrite as its main phase to largely improve hole expandability. The steel is designed to be composed of a single structure, and thereby the strain and stress concentration described above are prevented from occurring and a high hole expansion ratio can be obtained.
However, the steel is designed to be composed of a single structure of bainite or bainitic ferrite, and thereby elongation deteriorates greatly and the achievement of elongation and hole expandability cannot be attained.
Further, in recent years, there have been proposed high-strength steel sheets in which ferrite excellent in elongation is used as a structure of a single structure steel and a strength increase is achieved by using carbide of Ti, Mo, or the like (for example, Patent Documents 2 to 4).
However, the steel sheet proposed in Patent Document 2 contains a large amount of Mo. The steel sheet proposed in Patent Document 3 contains a large amount of V. Further, the steel sheet proposed in Patent Document 4 needs to be cooled in the middle of rolling for making crystal grains fine. Therefore, there is a problem that the alloy cost and the manufacturing cost increase. Further, even in this steel sheet, ferrite itself is largely increased in strength, and thereby the elongation deteriorates. The elongation of the single structure steel composed of bainite or bainitic ferrite is excellent, but the elongation-hole expandability balance is not necessarily sufficient.
Further, in Patent Document 5, there has been proposed a dual phase steel sheet in which in a DP steel, bainite is used in place of martensite and a strength difference between structures of ferrite and bainite is decreased, to thereby increase hole expandability.
However, as a result that an area ratio of the bainite structure was increased in order to secure strength, the elongation deteriorated and the elongation-hole expandability balance was not sufficient.
Further, in Patent Documents 7 to 9, there have been also proposed steel sheets in which ferrite in a DP steel is precipitation-strengthened and thereby a strength difference between ferrite and hard structure is decreased.
However, in this technique, Mo is an essential element to cause a problem that the manufacturing cost increases. Further, even though ferrite is precipitation-strengthened, the strength difference between ferrite and martensite being a hard structure is large, resulting in that a high hole expandability improving effect is not obtained.
On the other hand, in order to turn a microstructure into a dual phase of ferrite and martensite, Si is often added to these DP steels for the purpose of promoting ferrite transformation. However, when Si is contained, a tiger stripe scale pattern called a red scale (Si scale) is generated on the surface of the steel sheet, so that it is difficult to apply the DP steel to various steel sheets used for highly-designed wheel discs required to have beautifulness.
In Patent Document 10, there has been disclosed a technique relating to a steel sheet capable of obtaining an excellent balance between elongation and hole expandability by controlling a martensite fraction in a DP steel to 3 to 10% in a steel sheet of 780 MPa grade or higher. However, 0.5% or more of Si is added, thereby making it difficult to avoid the Si scale pattern, so that it is difficult to apply the technique to various steel sheets used for highly-designed wheel discs required to have beautifulness.
With regard to this problem, there has been disclosed a technique of a high-tensile hot-rolled steel sheet capable of suppressing occurrence of red scales by suppressing the added amount of Si to 0.3% or less and further obtaining high strength and excellent stretch flangeability by adding Mo and making precipitates fine (for example, Patent Documents 11 and 12).
However, in steel sheets having had the above-described technique disclosed in Patent Documents 11 and 12 applied thereto, the added amount of Si is about 0.3% or less, but it is difficult to sufficiently suppress occurrence of red scales, and further adding 0.07% or more of Mo being an expensive alloy element is essential, so that there is a problem that the manufacturing cost is high.
Further, in Patent Document 13, there has been disclosed a technique of avoiding occurrence of red scales by defining the upper limit of the content of Si. However, there is no technical disclosure on notch fatigue property.
Further, in Patent Document 14, there has been disclosed a technique of improving a low cycle fatigue property by adding Al. However, there is no technical disclosure on notch fatigue property being a fatigue property under stress concentration.